1. RNA and Protein Synthesis
7.2 Transcription & Gene Expression
 RNA and Protein Synthesis

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Essential Idea Information stored as a code in DNA is copied onto mRNA.
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Understanding
Transcription occurs in a 5’ to 3’ direction.
Nucleosomes help to regulate transcription in eukaryotes.
Eukaryotic cells modify mRNA after transcription.
Splicing of mRNA increases the number of different proteins an organism can produce.
Gene expression is regulated by proteins that bind to specific base sequences in DNA.
The environment of a cell and of an organism has an impact on gene expression.
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IB Assessment Statement
State that transcription is carried out in a 5’→ 3’ direction.
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Direction of Transcription:
The 5' end of the nucleotide is added to the 3' of the already existing mRNA chain.
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The sense strand has the same base sequence as the transcribed mRNA except that the base thymine is replaced by the base uracil. The anti-sense strand acts as the template for the transcription of mRNA. The RNA nucleotides are polymerised along the sugar phosphate backbone by RNA polymerase.
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IB Assessment Statement
Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator.
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Transcription in Prokaryotes
The Promoter region allows the binding of RNA polymerase. The RNA polymerase is then able to:
Find the anti-sense strand.
Find the start for transcription.
Know the direction of transcription.
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Transcription in Prokaryotes The Promoter region allows the binding of RNA The hydrogen bonds between the bases of the DNA helix are opened up by DNA helicase. The bases of the anti-sense strand ('3 to 5' for DNA) are exposed progressively. RNA nucleotides complementary base pair with the anti-sense nucleotide bases. The free nucleotides (nucleoside triphosphates) are based on RNA. The sugar is the pentose ribose and there are four different nitrogen bases. The nucleotides are adenine, guanine, cytosine and uracil.
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Transcription in Prokaryotes The RNA polymerase forms covalent bonds between the nucleotides. Free energy is released from the oxidation of the nucleoside triphosphates to form the bond. The bonds are formed by joining the 5' of the free nucleotide to the 3' end of the nucleotide already part of the mRNA chain. The RNA polymerase works along the nucleotides completing the pentose-phosphate backbone.
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Transcription in Prokaryotes
The mRNA builds up with the RNA polymerase moving along the anti-sense strand joining the nucleotides.
As with the other biochemical processes considered in the syllabus there are additional factor involved in transcription. These are not required for the examination
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Transcription in Prokaryotes
The RNA polymerase reaches the terminator and the RNA polymerase stops.
The mRNA is complete
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Transcription in Prokaryotes Various factors result in the RNA polymerase being released and will return to catalyze another mRNA. The mRNA itself is released from the antis-sense strand. The mRNA strand in prokaryotes can be use straight away unlike the eukaryotic mRNA which requires further modification (see next slide)
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IB ASSESSMENT STATEMENT State that eukaryotic RNA needs the removal of introns to form mature mRNA
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18. The introns are cut out of RNA molecules.
RNA Editing
mRNA Editing
The introns are cut out of RNA molecules.
The exons are the spliced together to form mRNA.
Exon
Intron
DNA
Pre-mRNA
mRNA
Many RNA molecules have sections, called introns, edited out of them before they become functional. The remaining pieces, called exons, are spliced together. Then, a cap and tail are added to form the final RNA molecule.
Cap
Tail

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mRNA Editing re-mRNA has been produced through transcription of the anti-sense strand as described for prokaryotic transcription. (a) The non coding introns are spliced out of the mRNA. The introns are broken down in the nucleus. (b) The remaining mRNA is called mature mRNA and is exported from the nucleus to the cytoplasm for translation into the polypeptide
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Animation on mRNA editing
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22. Translation
Messenger RNA is transcribed in the nucleus, and then enters the cytoplasm where it attaches to a ribosome.
Nucleus
During translation, or protein synthesis, the cell uses information from messenger RNA to produce proteins. The cell uses all three main forms of RNA during this process.
mRNA

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How are genes regulated? – TWO METHODS
Histone proteins in nucleosomes help to regulate transcription in eukaryotes by modifying it structure chemically which enhance or inhibit the expression of genes (transcription)
Regulatory Proteins (i.e.hormones)that bind to specific DNA Sequences which enhance or inhibit the expression of genes (transcription)
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Gene Regulation & Expression Nucleosomes help to regulate transcription in eukaryotes.
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METHOD 1: Nucleosomes & Gene Expression (the regulation of transcription)
Nucleosomes have proteins called histones.
Chemical modification of these histones are an important factor is determining whether a gene is expressed or NOT.
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27. Method 1: Chemical Modification of Histones & Gene expression
Types of modifications of histones:
Addition of acetyl group to a histone tail
Addition of a methyl group to a histone tail
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28. Method 1: Chemical Modification of Histones & Gene expression
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29. Method 1: Chemical Modification of Histones & Gene expression
Chemical Modification of histones can either activate or deactivate genes by increasing or decreasing the accessibility of the gene to transcription factors
Acetyl group: neutralizes the positive charge on histones, making DNA less tightly coiled–> increases transcription
Methyl group: maintains positive charge on histones, making DNA tightly coiled –> decreases transcription

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32. Method 1: Chemical Modification of Histones & Gene expression

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Method 2: Gene expression is regulated by proteins that bind to specific base sequences in DNA.
DNA SEQUENCES (in which regulatory proteins can bind)
Enhancers: regulatory sequences on DNA which increase the rate of transcription when proteins bind to them.
Silencer : sequences on DNA which decrease the rate of transcription when proteins bind to them.
Enhancers & Silencers are unique for each gene.
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Gene Expression by regulatory proteins. _L4CA OPzo
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Epigenetics: The environment of a cell and of an organism has an impact on gene expression. s/intro/
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36. Environmental Affects of Genetic Expression
The term epigenetics refers to heritable changes in gene expression (active versus inactive genes) that does NOT involve changes to the underlying DNA sequence;
Epigenetic change is a regular and natural occurrence but can also be influenced by several factors:
Including age,
the environment/lifestyle,
and disease state.
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37. Environmental Affects of Genetic Expression
Epigenetic modifications can manifest as commonly as the manner in which cells terminally differentiate to end up as skin cells, liver cells, brain cells, etc. Or, epigenetic change can have more damaging effects that can result in diseases like cancer.
At least TWO systems in which gene expression is modified including:
DNA methylation
histone modification
New and ongoing research is continuously uncovering the role of epigenetics in a variety of human disorders and fatal diseases.
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